The polyphenylene ethers and processes for their preparation are known in the art and described in numerous publications including Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875; J. G. Bennett and G. D. Cooper, U.S. Pat. No. 3,639,656; Cooper and Bennett, U.S. Pat. Nos. 3,642,699 and 3,661,848 and in copending applications of Cooper, Ser. No. 718,836, filed Aug. 30, 1976; and of Bennett and Cooper, Ser. No. 718,834, filed Aug. 30, 1976, all of which are incorporated herein by reference.
The processes most generally used to produce the polyphenylene ethers involve the self-condensation of a monovalent phenol in the presence of an oxygen-containing gas and a catalyst comprising a metal-amine complex.
At the conclusion of the reaction, the reaction solutions obtained, e.g., by oxidizing 2,6-xylenol with a copper-amine catalyst, are extracted with aqueous mineral acid or acetic acid or a mixture of water and carbon dioxide to remove the metallic component of the catalyst and the amine, before isolation of the polymer by precipitation with an antisolvent, such as methanol. It is imporant to remove the metallic catalyst residue from the reaction solution (and the polymer) because contamination of the polymer by metallic residues results in discoloration and degradation.
In J. G. Bennett and G. D. Cooper, U.S. Pat. No. 3,838,102, assigned to the same assignee herein, is described a new method which is extremely effective for removing metallic residues from polyphenylene ether reaction mixtures. The method yields polymer with very low metal content after precipitation either conventionally by adding an antisolvent or by total isolation procedures. The method of U.S. Pat. No. 3,838,102 involves adding a polyfunctional compound to the reaction mixture, the compound being capable of selectively complexing with the metallic component of the catalyst, to decompose the catalyst complex and to form a water soluble, extractable composition of the metal and the polyfunctional compound.
Molecular weight control problems are also encountered, however. It is known that, when polyphenylene ether reaction mixtures are allowed to stand for appreciable periods before isolation of the polymer, the intrinsic viscosity (I.V.) of the polyphenylene ether is reduced; the extent of the I.V. drop depends on the time between reaction and isolation, the temperature of the mixture, and probably on the conditions used in preparing the polymer. In typical large scale operations, with the reaction mixture held at 50.degree. C., the I.V. drop is usually more than 0.1 dl./g. and drops greater than 0.2 dl./g. are not uncommon.
In practice, an attempt is made to compensate for this degradation by adjusting the polymerization conditions to prepare a polymer of substantially higher I.V. than that desired in the final product, so that after the I.V. drop between reaction and isolation, the intrinsic viscosity will fall in an acceptable range. However, this method is expensive, requiring more catalyst than would otherwise be necessary, and difficult to control, because the amount of the I.V. drop may vary widely, especially when upsets anywhere in the system cause an especially long delay in isolating the polymer. A method for preventing or minimizing the I.V. drop in polyphenylene ether reaction mixtures would, therefore, be extremely useful.
German Patent Publication No. 2430130, Jan. 23, 1975, discloses a method for stabilizing I.V. in polyphenylene ether reaction mixtures by adding a mixture of a dihydric phenol such as hydroquinone or catechol, or a benzoquinone, and a mild reducing agent, such as sodium sulfite. The publication teaches that the dihydric phenol should be used in an amount greater than two moles per gram-atom of the copper or other metal catalyst used in the polymerization, and preferably, at a level of at least 5 moles per gram-atom.
Unexpectedly, it has now been found that by treating the polyphenylene ether reaction mixture with a combination of the dihydric phenol/reducing agent and a chelating agent for the metal catalyst, such as a salt of ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA), intrinsic viscosity degradation may be prevented with much smaller amounts of the dihydric phenol than are taught to be necessary in the German publication, above-mentioned. Surprisingly, neither the chelating agent nor the low amounts of dihydric phenol/reducing agent suitable for use in the combination when used alone have any appreciable effect in reducing the rate or extent of the I.V. drop. Illustratively, stabilization has been achieved at catechol: copper ratios of 0.36:1. This is less than one-fifth the minimum amount said to be necessary in the German publication. This ratio is further reducible. A more than merely additive effect is seen by using the combination. Obviously, such a reduction in the amount of the dihydric phenol is important because of the cost since the dihydric phenol is by far the most expensive component of the stabilizer system. Moreover, also avoided or minimized are problems in waste water disposal caused by the bactericidal activity of the dihydric phenols. It is a common practice to hold aqueous waste in treatment ponds, where the organic components are digested by bacteria and other microorganisms to reduce the chemical oxygen demand of the water to an acceptable level before it is discharged into the streams. Hydroquinone and catechol are about 20 times as lethal to typical microorganisms as is phenol (Water Quality Criteria, State of California Water Control Board, Publication 3A, 2nd edition).
Obviously, high concentrations of dihydric phenols would kill the microorganisms and prevent the treatment pond from doing its job. Thus, the use of less of this component is a substantial beneficial advantage.